thread_scoped_ref

A library that is similar to a thread local storage but allows to store references / dyn Trait within a scope. It can be used to 'inject' references (if you don't own the data and Rc/Arc is not possible) into something you don't control entirely (e.g. a function you provide that gets called by a library you don't own).

It's also very similar to https://github.com/alexcrichton/scoped-tls, with those differences:

• thread_scoped_ref works with traits / unsized types.
• thread_scoped_ref does not panic when calling ScopedKey::with, instead calls the closure with None.

According to scoped-tls there once was something similar in the old rust standard library (quote from scoped-tls):

A Rust library providing the old standard library's scoped_thread_local! macro as a library implementation on crates.io.

... and another library that seems to do the same (and a bit more, like mutable references) https://crates.io/crates/scoped-tls-hkt - it seems that we both worked on this at almost the same time (the author of scoped-tls-hkt released it last week; 2020-04-18). You should definitely check that out too.

Example use case:

.
          +----- (set) ---------> &Data <------- (access/read) ----------+
          |                                                              |
+---------+------------+    +--------------------------------------------|-------------+
| Data                 |    | External library                           |             |
| (huge/context/no Rc) |    |                                            |             |
+----------------------+    |                                +-----------+------+      |
                            |              ---- (calls) ---> | Your function    |      |
                            |                                +------------------+      |
                            +----------------------------------------------------------+

More information

• crates.io
• Documentation
• Tests

Usage

Cargo

[dependencies]
thread-scoped-ref = "0"

Example

use thread_scoped_ref::{thread_scoped_ref, scoped, with};
use std::collections::HashMap;

thread_scoped_ref!(SOME_ENV_VALUES, HashMap<String, String>);

/// It's not possible to pass `&HashMap<String, String>` to this function since it's called
/// by some library you don't control...
fn read_env_value() {
  // ... so we read from the static 'SOME_ENV_VALUES'.
  with(&SOME_ENV_VALUES, |maybe_env_values| {
    // don't "unwrap" in reality: Since `maybe_env_values` will be `None` if not
    // called within a scope!
    let env_values = maybe_env_values.unwrap();
    assert_eq!("true", env_values.get("delete_entire_ssd").unwrap());
  });
}

 /// An external library you don't control or generated code.
fn external_library(function_ptr : fn()) {
   function_ptr();
}

let mut env_values = HashMap::default();
env_values.insert("delete_entire_ssd".to_string(), "true".to_string());
// Create a scope. Note: We only need a reference to `env_values` (no move required).
scoped(&SOME_ENV_VALUES, &env_values, || {
  external_library(read_env_value);
});

License

Licensed under either of

• Apache License, Version 2.0, (http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (http://opensource.org/licenses/MIT)

at your option.